1. Field of the Invention
The present invention relates to a method of detecting an alignment mark for aligning a photomask during exposure processing and a method of manufacturing a printed circuit board including the exposure processing.
2. Description of the Background Art
In a printed circuit board, a conductor layer having a given pattern is formed on an insulating layer. Application of a photosensitive resist (hereinafter abbreviated as a resist), exposure processing and development processing for patterning the conductor layer, for example, are performed in manufacturing steps of the printed circuit board. The resist is applied on the conductor layer, thereby forming a resist layer on the conductor layer, for example. In the exposure processing, a surface of the resist layer is irradiated with light through a photomask. Thus, the resist layer is exposed in a pattern of the photomask. The development processing is then performed, so that the resist layer having the given pattern (hereinafter referred to as a resist pattern) is formed on the conductor layer.
In this state, a region of the conductor layer excluding a region below the resist pattern is removed by etching, for example, and the resist pattern is subsequently removed. In this manner, the conductor layer is formed in the given pattern.
During the foregoing exposure processing, it is necessary to detect a region to be exposed (hereinafter referred to as an exposure region) of the resist layer on a base material such as the conductor layer, and align the photomask with the exposure region. For aligning the photomask, an alignment mark for exposure is provided in the base material, and the photomask is aligned based on the alignment mark (see JP 2886675 B, for example).
When the photomask is aligned in the foregoing exposure processing, the base material is photographed by a camera, and an image including the alignment mark is processed, so that a position of the alignment mark is detected.
In this case, the alignment mark needs to exist within an area of the image obtained by the camera. Usually, the base material is transported such that the alignment mark for the current exposure processing exists within the area of the image based on information related to the position of the base material in the preceding exposure processing. However, the alignment mark may not exist within the area of the image obtained by the camera or only part of the alignment mark may exist within the area of the image due to an error in a transport speed or the like.
In such a case, it is necessary to move the base material by a transport device, photograph the base material by the camera and search for the alignment mark on the image in a repetitive manner until the alignment mark exists within the area of the image obtained by the camera. Therefore, detection of the alignment mark takes time, resulting in a longer time required for the exposure processing.
When the imaging area is extended by lowering the imaging magnification of the camera, the alignment mark more easily exists within the area of the image. In this case, however, the detection accuracy of the position of the alignment mark is reduced, thus leading to lower detection accuracy of the exposure region. This results in lower alignment accuracy of the photomask.